1. Field of the Invention
This invention relates to an electronic tuning circuit for AM receiver adapted to receive desired broadcast waves with the aid of the electronic tuning circuit using variable capacitance diodes.
2. Description of the Prior Art
In order to have a better understanding of the present invention, description will first be made of a conventional electronic tuning circuit for AM receiver with reference to FIG. 9 of the accompanying drawings. As shown in FIG. 9, the conventional electronic tuning circuit comprises an antenna tuning circuit 12, a high-frequency amplifier circuit 13, a local oscillator circuit 15; and the AM receiver includes, in addition to such an electronic tuning circuit, a mixer circuit 15 provided at the back thereof, an intermediate-frequency amplifier circuit, a detector circuit, a low-frequency amplifier circuit and so forth. Indicated at 16 is a tuning voltage source 16 which is arranged to apply a tuning voltage Va to the antenna tuning circuit 12 and high-frequency amplifier circuit 13 and to apply an oscillation frequency controlling voltage Vb to the local oscillator circuit 14. L11 to L13 are tuning coils; C11 to C13 are trimmer capacitors; D11 to D13 are variable-capacitance diodes; C21 to C23 are capacitors; and C31 is a padding capacitor. With the conventional AM receiver, tracking adjustment is effected at three points in the receiving frequency band by adjusting the tuning coils L11 to 13 and trimmer capacitors C11 to C13.
A more detailed explanation of the conventional tracking adjustment is this: The local oscillator circuit is first adjusted; subsequently, the antenna tuning circuit and high-frequency amplifier circuit are adjusted; and finally tracking error at the center point of the receiving frequency band is confirmed. Generally, adjustment in the lower frequency range is effected by means of the tuning coils, while adjustment in the higher frequency range is effected by means of the trimmer capacitors. Thus, the conventional electronic tuning circuit for AM receiver is disadvantageous in that a number of components for effecting tracking adjustment are mounted so that a correspondingly increased number of steps are involved in the tracking adjustment, which leads to an increase in the manufacturing cost.
Another disadvantage of the conventional electronic tuning circuit is such that in case tracking adjustment cannot be effected, it is required either that all the mounted components be discarded or that adjustment be effected after the components have been replaced with new ones, due to the fact that tracking adjustment is effected after the tuning circuit at the front end portion of the AM receiver and succeeding IF amplifier and so forth have been mounted on a printed circuit board.